In an image forming apparatus using an electrophotographic process, a toner image is fixed onto a sheet by heating. However, moisture evaporates from the sheet having been heated in the process of fixing. This causes an uneven distribution in the percentage of water content in the sheet, with the result that sheet bending known under the name of “lenticulation” occurs.
To avoid such lenticulation, a sheet humidifying technology has been developed to supply water to the sheet with an image formed thereon. In such a sheet humidifying process, when a sheet is to be humidified, the entire surface of the sheet must be uniformly humidified. Uniform humidifying processes are proposed in the Unexamined Japanese Patent Application Publication No. 2006-8282 and U.S. Pat. Nos. 5,832,359 and 5,895,154.
In the Unexamined Japanese Patent Application Publication No. 2006-8282, water is supplied to the surface of a humidifying roller so that sheets are humidified, when sheets are held and conveyed by a pair of humidifying rollers. A regulating device for regulating the amount of supplied water is built into a water-supplying device for supplying the humidifying roller with water.
In the proposal made in the U.S. Pat. No. 5,832,359, when sheets are held and conveyed by a pair of humidifying rollers, water is supplied to the surface of a humidifying roller so that the sheets are humidified. The water layer on the surface of the roller for supply water to the humidifying roller is optically detected. Based on the result of detection, water supply is controlled, whereby uniform humidifying is achieved.
The U.S. Pat. No. 5,895,154 proposes a sheet humidifier wherein sheets are humidified by water supplied to the surface of a humidifying roller when the sheets are held and conveyed by a pair of humidifying rollers, and wherein the surface of the humidifying roller is made of fiber.
When a sheet humidifier has been used for a long time, the problem is a failure in ensuring uniform humidifying because of the deteriorated surface of the humidifying roller. The Unexamined Japanese Patent Application Publication No. 2006-8282, U.S. Pat. Nos. 5,832,359 and 5,895,154 disclose inventions of uniformly humidifying all the sheet surfaces. However, the Unexamined Japanese Patent Application Publication No. 2006-8282, U.S. Pat. Nos. 5,832,359 and 5,895,154 fail to consider the problem of uniform humidification being disabled by long-term operations.